1. Field of the Invention
The present invention relates to an offset adjusting apparatus.
2. Description of the Related Art
An offset adjusting apparatus is used for an apparatus that removes a DC offset included in an analogue signal. For example, Japanese Patent Application Laid-Open Publication No. 2004-348908 discloses an offset adjusting apparatus that removes an offset of an RF signal in a circuit that generates an RF signal from a signal output from an optical pickup of an optical disk apparatus.
FIG. 14 depicts a configuration of an offset adjusting apparatus equivalent to an apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2004-348908. An offset adjusting apparatus 100 includes an operational amplifier 110, resistors 111, 112, a low-pass filter (LPF) 113, an A/D converter 114, a voltage controlling unit 115, a D/A converter 116, and a CPU 120.
An input signal VIN input to the offset adjusting apparatus 100 is a signal output from an optical pickup, for example. Since the signal output from the optical pickup is weak, the offset adjusting apparatus 100 outputs an output signal VO acquired by amplifying the input signal VIN with a gain corresponding to a ratio of resistance values of the resistors 111 and 112. When the resistance value of the resistor 111 is R1 and the resistance value of the resistor 112 is Rf, the gain in the offset adjusting apparatus 100 is represented as Rf/R1. To set the amplitude level of the output signal VO to a predetermined level, the resistor value Rf of the resistor 112 is adjusted under the control of the CPU 120 to change the gain of the offset adjusting apparatus 100.
The input signal VIN includes an offset due to the characteristics of the optical pickup, etc. The operational amplifier 110 also generates an offset. If the offset stays in the output signal VO, subsequent processes may be affected. Therefore, the offset adjusting apparatus 100 controls a voltage applied to the + input terminal of the operational amplifier 110 to remove the offset included in the output signal VO.
The operation of removing the offset in the offset adjusting apparatus 100 will be described. First, the LPF 113 detects a DC level of the output signal VO. The detected DC level is converted by the A/D converter 114 into a digital signal, which is input to the voltage controlling unit 115. Under the control of the CPU 120, the voltage controlling unit 115 outputs the control data for controlling the voltage applied to the + input terminal of the operational amplifier 110 after increasing or decreasing the control data by one step such that the offset of the output signal VO is removed. The D/A converter 116 generates a voltage VDA acquired by analog conversion of the control data output from the voltage controlling unit 115 and applies the voltage VDA to the + input terminal of the operational amplifier 110. Until the offset of the output signal VO is removed, i.e., until the DC level of the output signal VO attains a predetermined target level, the voltage controlling unit 115 repeatedly changes the control data.
In the offset adjusting apparatus 100, the relation of the following equation (1) is satisfied.
                                                        V              IN                        -                          V              DA                                            R            1                          =                                            V              DA                        -                          V              O                                            R            f                                              (        1        )            
From equation (1), the output signal VO is represented by the following equation (2).
                              V          o                =                                            V              DA                        -                                                            R                  f                                                  R                  1                                            ⁢                              (                                                      V                    IN                                    -                                      V                    DA                                                  )                                              =                                                    (                                  1                  +                                                            R                      f                                                              R                      1                                                                      )                            ⁢                              V                DA                                      -                                                            R                  f                                                  R                  1                                            ⁢                              V                IN                                                                        (        2        )            
Therefore, a change amount ΔVO of the output signal VO corresponding to a change amount ΔVDA of the voltage VDA is represented by the following equation (3).
                              Δ          ⁢                                          ⁢                      V            o                          =                              (                          1              +                                                R                  f                                                  R                  1                                                      )                    ⁢          Δ          ⁢                                          ⁢                      V            DA                                              (        3        )            
Rf/R1 included in equation (3) represents the gain in the offset adjusting apparatus 100. That is, ΔVO becomes large when the gain of the offset adjusting apparatus 100 is large and ΔVO becomes small when the gain of the offset adjusting apparatus 100 is small. In other words, the offset adjustment accuracy in the offset adjusting apparatus 100 is fluctuated by the gain.
FIG. 15 depicts an example of an offset adjustment sequence for the output signal VO in accordance with the gain of the offset adjusting apparatus 100. It is assumed that the adjustment of the offset is started at time T1 and that the voltage VDA is reduced stepwise under the control of the voltage controlling unit 115. When the voltage VDA is reduced by one step, the change amount of the DC level of the voltage VO is changed in accordance with the gain (Rf/R1) of the offset adjusting apparatus 100 as described above. Therefore, if the gain (Rf/R1) of the offset adjusting apparatus 100 is large, the adjustment accuracy of the offset is deteriorated. As a result, a difference becomes larger between the DC level of the output voltage VO and the target level VREF and the result of the offset adjustment may also be deteriorated.
The following equation (4) is derived from equation (2) as a condition of removing the offset of the output signal VO.
                              V          DA                =                                            R              f                                                      R                1                            +                              R                f                                              ⁢                      V            IN                                              (        4        )            
When the gain (Rf/R1) of the offset adjusting apparatus 100 is represented by G, the condition of removing the offset of the output signal VO can be represented by the following equation (5).
                              V          DA                =                                                                              R                  1                                ⁢                G                                                              R                  1                                +                                                      R                    f                                    ⁢                  G                                                      ⁢                          V              IN                                =                                                    G                                  1                  +                  G                                            ⁢                              V                IN                                      =                                          1                                                      1                    G                                    +                  1                                            ⁢                              V                IN                                                                        (        5        )            
Assuming that the dynamic range of the voltage VDA is −A≦VDA≦A, the following equation (6) indicates an offset adjustment allowable range, which is a range of the input signal VIN enabling removal of the offset of the output signal VO.
                                          -                          (                              1                +                                  1                  G                                            )                                ⁢          A                ≤                  V          IN                ≤                              (                          1              +                              1                G                                      )                    ⁢          A                                    (        6        )            
As can be seen from equation (6), the input offset adjustment allowable range of the offset adjusting apparatus 100 is reduced as the gain G increases. Therefore, the offset of the output signal VO may not be removed in the offset adjusting apparatus 100 if the gain is increased.
The present invention was conceived in view of the above problems and it is therefore the object of the present invention to provide an offset adjusting apparatus having offset adjustment accuracy and offset adjustment allowable range that can be controlled regardless of the gain.